The present invention relates to a prepreg comprising a resin-impregnated, sheet-like, fiber-reinforced material that is thermal pressure adhered to a graft copolymer sheet-like preform, and more specifically to a prepreg having excellent high frequency signal transmission characteristics and excellent heat cycle resistance, and a conductive layer-laminated substrate for a printed wiring board manufactured by using the prepreg.
In order to increase the information transmission rates, communication devices and electronics devices use signals in high-frequency bands of from megahertz bands to gigahertz bands. However, the higher the frequency of an electric signal is, the more transmission loss occurs. There has been demanded an electrical insulating material that is applicable for transmitting high-frequency signals such as in gigahertz bands and that has excellent high frequency signal transmission characteristics. The transmission loss of a circuit contacted with an insulating material includes conductor loss that is determined by the shape, the skin resistance, and the characteristic impedance of the circuit (conductor); and dielectric material loss that is determined by the dielectric characteristics of an insulating layer (dielectric) around the circuit. The transmission loss may be radiated as the dielectric material loss from high-frequency circuits, and may be a factor that causes malfunctions of electronics devices. The dielectric material loss increases in proportion to the product of dielectric constant (∈) and dielectric loss (tan δ) of a material. In order to decrease the dielectric material loss, it is necessary to use a material that has both small dielectric constant and small dielectric loss.
As a material that has small dielectric constant and small dielectric loss, polymeric insulating materials, such as insulating thermoplastic resins and insulating thermosetting resins, are known. As the insulating thermoplastic resins, polyolefins, liquid crystal resins, and fluorinated resins have been proposed. As the insulating thermosetting resins, there have been proposed unsaturated polyester resins, polyimide resins, epoxy resins, bismaleimide-triazine resins (BT resins), curable polyphenylene oxides, and curable polyphenylene ethers.
In particular, as a material that has small dielectric constant and small dielectric loss in high-frequency bands, there are known polytetrafluoroethylene (see Japanese Patent Laid-Open Publication No. 11-087910), liquid crystal resins (see Japanese Patent Laid-Open Publication No. 09-23047), and modified polyolefin resins (see WO99/10435). The polytetrafluoroethylene particularly exhibits excellent high frequency signal transmission characteristics, however, has drawbacks of difficulties in processing and high cost. The liquid crystal resins are excellent in cost and processability, but, lacks high frequency signal transmission characteristics.
The modified polyolefin resins are appropriate in terms of cost, processability and high frequency signal transmission characteristics, and thus have been expected as useful electrical insulating materials. However, the modified polyolefin resins are highly resistant to solvents. Therefore, the modified polyolefin resins cannot be used in method for manufacturing a prepreg by dissolving a resin in a solvent and impregnating the solvent into a sheet-like, fiber-reinforced material such as a glass cloth.
WO99/10435 discloses a method that enables use of the modified polyolefin resins for manufacturing a prepreg. Specifically, there is disclosed a method of forming films with a modified polyolefin resin, sandwiching a sheet-like, fiber-reinforced material between the films and conducting thermal pressure adhesion.
This method requires that a glass cloth is impregnated with a molten resin. However, when the modified polyolefin resins are crosslinked resins, the resins do not show steep increase of flowability in temperatures more than or equal to melting points. Therefore, it is not easy that the glass cloth in a prepreg is fully impregnated with the modified polyolefin resins, and voids can remain within the glass cloth. Application of heat cycles such as repetition of cooling and heating to a prepreg or a conductive layer-laminated substrate for a printed wiring board can generate cracks in a resin layer due to repetition of swelling and shrinking of the confined voids.
In order to reduce the generation of cracks, an effective method is to impregnate a resin that behaves in an analogous fashion to a resin constituting a prepreg in behavior of swelling and shrinking into voids of a glass cloth. Japanese Patent Laid-Open Publication No. 10-265592 discloses a method for manufacturing a prepreg by subjecting a glass cloth into which a varnish obtained by dissolving a styrene-ethylene-butylene-styrene copolymer or a modified polyphenylene ether in a solvent is impregnated and a film of syndiotactic polystyrene, which is a thermoplastic resin, to thermal pressure adhesion. Use of this method makes it possible to impregnate the voids of the glass cloth and reduce swelling of the prepreg at the time of high temperature heating.
According to the method disclosed in Japanese Patent Laid-Open Publication No. 10-265592, a prepreg is manufactured by subjecting a syndiotactic polystyrene film to thermal pressure adhesion with a glass cloth in temperatures more than or equal to the melting point of syndiotactic polystyrene. However, the syndiotactic polystyrene melts and exhibits high flowability at the time of thermal pressure adhesion, and it is difficult to control a prepreg to have a desired thickness. In addition, in order to prevent sagging due to heat caused by flowability of molten syndiotactic polystyrene, it is necessary to use large amounts of glass-fiber reinforcements. However, the more the amount of the glass-fiber reinforcements in a prepreg become, the more areas of contact between the inorganic material and the organic material that have considerably different rates of swelling increase. In this case, the inorganic material tends to break away from the organic material, and thus heat cycle resistance certainly deteriorates even though apparent voids are reduced.
Japanese Patent Laid-Open Publication No. 10-265592 discloses use of a resin to which a polar functional group is introduced such as maleic acid for the purpose of reducing the voids. However, when affinity between a resin and a glass-fiber reinforcement is enhanced by introducing a polar group, the interaction between the polar group and the glass-fiber reinforcement deteriorates dielectric constant and dielectric loss of a prepreg in high-frequency bands rather than increases dipole moment of the resin. Therefore, the prepreg according to Japanese Patent Laid-Open Publication No. 10-265592 is not suitable for application in a printed wiring board for high-frequency bands.